Tubing is produced at the desired diameter by drawing larger diameter tubing through a die having a tapered orifice therein with a floating die plug positioned within the larger diameter tubing as it enters the tapered orifice. The process is repeated with dies having smaller orifices and smaller die plugs until the desired outer diameter is achieved. With each pass through a die, the diameter and wall thickness of the tubing is reduced and the length of the tubing is increased.
The tubing both before and after being drawn through the die is formed into coils, say, up to 125 inches in diameter, depending upon the particular materials and equipment. The coiled tubing is passed to the die from a rotating pay-off reel or pay-off tray. The tubing is pulled through the die by being wrapped upon a rotating drawblock. The axes of rotation of the pay-off tray and the drawblock may be vertical or horizontal. This invention relates, for the most part, to vertical drawblocks, i.e., having vertical axes of rotation. The tubing forms a coil as it is wrapped upon the drawblock. The tubing cannot be overlapped upon the drawblock for the obvious reason that it would be crushed out of shape. Two types of drawblocks are used: One, known as non-continuous, has a long axial length winding drum. The die box is shifted in the axial direction relative to the drum as the tubing is wrapped upon it to prevent overlap. The length of the tubing wrapped upon the drawblock is limited by the length and diameter of the winding drum. Another, type of drawblock, known as continuous, is arranged so that as the tubing is wrapped upon the drum of the drawblock (in this case a capstan), the wraps are urged down to one end where they fall off the capstan. After the initial wraps are established upon the drum, the die remains in a fixed axial position relative to the capstan. Each new wrap is laid upon the same position on the capstan and is immediately moved (plowed) away to make room for the next wrap. For every wrap added to the drawblock, another falls off the capstan into a receiving tray. In the case of a continuous drawblock, the rotating speeds of the pay-off tray and the receiving tray are synchronized with the capstan to suit the coil diameters at each of these stations.
The diameter of the drawblock is related to the entry and finished size of the tubing. For example, for a three inch entry tubing diameter finishing down to 1/4 inch, the diameter of the drawblock may be about 84 inches and for smaller entry tubing diameter finishing down to 1/8 inch, the diameter of the drawblock may be 60 inches or 40 inches. Typically, tubing is pulled through the die and wrapped upon the drawblock at speeds up to 4000 feet per minute.
Prior to drawing a tube through the die, the operator first deposits lubricant, then inserts the die plug in the tube near the end to be fed to the die. As a practical matter, the operator dimples the tubing to restrict the position of the plug a desired distance from the end of the tube. The operator then "points" (tapers) the end of the tube in a machine designed especially for that purpose. He can then start the pointed end of the tube through the die. A gripper fixed to the capstan is used to secure the end of the tube passed through the die so that the tube will be pulled through the die as the capstan is rotated. The operator then initiates rotation of the capstan. In the continuous drawblock, the tubing is automatically cut free from the gripper after a number of wraps, say 5 to 10, have been placed upon the capstan. Thereafter, as each new wrap is added to the capstan another falls away.
The die is carried by a die box which is a stage for holding the die and a plurality of guide wheels having grooved rims for guiding the entry tubing into the die. The spacing of the guide wheels is adjustable to accommodate entry tubing of various diameters. Likewise, the die is interchangeable in the die box to accommodate the size of the entry tubing.
The die box is mounted to provide at least three modes of freedom. The die box must be translated in the direction of the axis of the capstan from a position opposite the gripper to the position where the wraps are continuously laid upon the capstan. This is usually a vertical translation of, say, 24 inches. The die box is also journaled to rotate about an axis parallel to the axis of the capstan (usually a vertical axis). It rotates from an angular position where tubing exiting from the die is aligned with the position of the gripper to an angular position where the tubing exiting the die is aligned with the tangent to the capstan where the tubing first contacts the capstan. This rotation amounts to about 5 to 20 degrees. Still further, the die box is journaled to rotate about an axis (usually a horizontal axis) perpendicular to the already described axis to permit the tubing to face the axial position on the drawblock where tubing is being wrapped upon the capstan. Hence, the die box rotates from a horizontal position at the time of griping (the start of draw position) to point downwards as the die box is being translated upward to the drawing position where it rotates back to the horizontal position. This rotation amount to about 15 to 25 degrees.
Prior art die boxes were provided with hydraulic pistons and cylinders for clamping the die box to restrict rotation about the axis parallel to the axis of the capstan and in the direction of the gripper. Also, the die box could be clamped about this axis in the nominal direction of the tangent to the drum where the tubing first contacted the capstan. However, the direction was only correct for one diameter of tubing being wrapped upon the capstan and was incorrect for all other diameters. Tubes pulled through the die when the direction of the tube exiting from the die was incorrect could be improperly formed with variable wall thickness or, worse yet, broken. For this reason, this axis was usually not clamped in the drawing position. Unfortunately, this also has its drawbacks. The action of the entry tubing uncoiling from the pay-off tray and the tubing wrapping upon the capstan could set up vibrations of the die box that affect the uniformity and surface of the finished tubing. More serious, if the die box were bumped or an imperfection in the tubing passed through the die, the die box could be jolted enough to cause a break in the tubing.